Sex hormone binding globulin levels across the adult lifespan in women--the role of body mass index and fasting insulin.

SHBG is a major carrier of androgens. In men, SHBG levels increase with age, while in women data are scant. There is evidence that body mass index (BMI) and fasting insulin influence SHBG concentration. Since low SHBG levels are predictors of insulin resistance and diabetes, understanding the relationship of SHBG with age, insulin, and BMI is important to gain insight into the role of SHBG as a cardiovascular risk factor in women. Differences in SHBG across adult life span and their relationship with insulin and BMI were evaluated in a representative cohort of 616 Italian women free of diabetes and not on hormone replacement therapy enrolled in the InCHIANTI Study. The relationship of SHBG with age, BMI, and fasting insulin levels was analyzed using linear regression and by loess smoother. Serum SHBG levels showed a U-shaped trajectory with age, declining from the 2nd to the 6th decade of life and increasing after the 6th decade (p<0.0001). Age-related trends for BMI and fasting insulin mirrored the trend observed for SHBG. After adjusting for fasting insulin, the relationship between log (SHBG) and age square was attenuated (beta coefficient from 0.00044 to 0.00039) and was further reduced after adjustment for BMI (from 0.00039 to 0.00028). SHBG levels show an age-related U-shaped trajectory. These changes mirror the age-related changes in BMI and fasting insulin, suggesting that BMI and insulin negatively influence SHBG concentration.